The peak-to-average power ratio (“PAPR”), also known as peak-to-mean power ratio (“PMPR”) or peak factor, is an important characteristic of multi-carrier transmitted signals. The peak of the signal can often be N times greater than the average power level. These large peaks cause intermodulation distortion which can result in an increase in the error rate. These distortions are brought about from the limitations inherent in a transmitting amplifier.
In order to prevent the transmitter amplifier from limiting (clipping), the average signal power must be kept low enough to keep the signal relatively linear through the amplifier. In order to transmit a high power signal, a high power amplifier is required which requires a large DC system power. A much higher power amplifier is required to transmit multi-carrier waveforms than for constant envelope waveforms. For example, using 64 carrier waveforms, a 40 dBm power amplifier would require about 15 dB of back off. Therefore, instead of operation at 40 dBm (10 watts) the amplifier is only capable of operating at 25 dBm (0.316 Watts). Thus in order to transmit at the desired 40 dBm, a 55 dBm (316 Watt) amplifier would be required. The associated power supply, power consumption, can be substantially increased. In addition, such large power requirements lead to associated increased space demands and heat dissipation requirements.
With the large amount of interest and activity with Orthogonal Frequency Division Modulation (“OFDM”), and in particular 802.11a and 802.11g communication technology, the PAPR problem is exaggerated. 802.11 with its use of complex waveforms requires highly linear RF amplifiers. Current 802.11 physical layer integrated circuits have not implemented PAPR reduction schemes. In particular, multi-tone OFDM typically requires greater than 10 dB power amplifier back-off because of a high peak-to-average power ratio. The net result of these factors is an increased DC power demand beyond that encountered with other 802.11 techniques. The effect may be less noticeable for short duty cycle signals, but can be significant for situations requiring continuous transmission of data.
OFDM, as mentioned above, is a method of transmitting data simultaneously over multiple equally-spaced and phase synchronized carrier frequencies, using Fourier transform processing for modulation and demodulation. The method has been proposed and adopted for many types of radio systems such as wireless Local Area Networks (“LAN”) and digital audio and digital video broadcasting. OFDM offers many well-documented advantages for multi-carrier transmission at high data rates, particularly in mobile applications. Specifically, it has inherent resistance to dispersion in the propagation channel. Furthermore, when coding is added it is possible to exploit frequency diversity in frequency selective fading channels to obtain excellent performance under low signal-to-noise conditions. For these reasons, OFDM is often preferable to constant envelope modulation with adaptive equalization and is arguably less complex to implement.
The principal difficulty with OFDM, as alluded to above, is that when the sinusoidal signal of the N carriers add mostly constructively, the peak envelope power is as much as N times the mean envelope power. If the peak envelope power is subject to a design or regulatory limit then this has the effect of reducing the mean envelope power allowed under OFDM relative to that allowed under constant envelope modulation. If battery power is a constraint, as is typically the case with portable equipment such as mobile consumer appliances, laptops, and sophisticated Department of Defense communication systems, then the power amplifiers required to behave linearly up to the peak envelope power must be operated inefficiently with considerable back-off from compression. Digital hard limiting of the transmitted signal has been shown to alleviate the problem but only at the cost of spectral sidelobe growth and consequential bit error performance degradation.
FIG. 1 illustrates 16 carriers in-phase with frequencies ranging from C hertz for carrier 101 to 16 C hertz for carrier 116, with the intermediate carriers having increasing frequencies with steps of C, characteristic of an OFDM signal. Each of the carriers as shown have a nominal maximum amplitude of one, however as seen in FIG. 2 the disparate effects of the carriers added in-phase are readily apparent. FIG. 2 shows the large peak amplitudes of the added carriers at around time sample 25 and around time sample 1575.
FIG. 3 illustrates the peak-to-average power ratio of the 16 carriers modulated in-phase. The large peak-to-average power ratios correspond to the large amplitude spikes illustrated in FIG. 2. The peak-to-average power ratio for FIG. 3 is generated according to the function:
            P      peak              P      avg        =            X      i      2        /                  (                              1            N                    ⁢                                    ∑              1              N                        ⁢                          X              i              2                                      )            2      where Xi is the signal sample amplitude at sample number i and N is the number of samples of the multi-carrier symbol.
These problems provide a clear motivation to find other solutions for controlling the peak to mean envelope power ratio of the transmitted signal. One solution offered uses block coding to transmit across the carriers only those poly-phase sequences with small PAPR; however, this entails an exhaustive search to identify the best sequences and requires large look-up tables for encoding and decoding.
Some techniques, such as spectral whitening, serve to reduce the peak-to-average ratio and allow the use of RF amplifiers closer to their 1 dB compression point, resulting in a decreased DC power demand. Some prior art solutions have used clipping or mapping to reduce the PAPR. However no solutions have employed or suggested a hybrid system, including selective mapping and soft clipping as is presented in this disclosure.
It is an object of the present disclosure to obviate the disadvantages of the prior art and present a novel system and method for reducing the peak-to-average power ratio of a signal for transmission in a multi-carrier communication system. One method sequences information data according to a data vector and modulates multi-carrier symbols with the sequenced data. The resultant modulated data signal's peak-to-average power ratio is measured and compared to a predetermined threshold. In the method, if the peak-to-average power ratio exceeds the predetermined threshold, the data is re-sequenced in accordance with a new data vector and repeats the modulation and comparison processes. Otherwise the modulated data signal is appended with a data map associated with the respective data vector and sampled. Those modulated data signal samples which exceed a predetermined range are clipped and the clipped modulated data signal is filtered, thereby reducing the PAPR ratio of the signal to be transmitted in a multi-carrier communication system.
It is a further object of the present disclosure to present a novel system and method, in a multi-carrier communication system, of transmitting data. An embodiment of a system and method includes sequencing the data according to one or more unique sequences, modulating one or more of the sequences of data and selecting one of the modulated sequences of data, based on the PAPR. The system and method further include filtering the selected modulated sequence of data to remove amplitude peaks outside a threshold band, and transmitting the filtered signal over the multi-carrier communication system.
It is another object of the present disclosure to present in a multi-carrier communication system with a linear amplifier, a novel system and method of preventing limiting of the amplifier. The novel system and method include sequencing data to be transmitted based upon the resultant PAPR from the modulation of the sequenced data. Also included is sampling the modulated sequenced data and truncating the samples which are outside a threshold, thereby forming a data signal that prevents limiting of the amplifier.
It is still another object of the present disclosure to present, in a multi-carrier communication system for transmitting data, a novel system and method for forming a data signal that reduces the required power of a transmitter. The novel system and method includes providing the data to be transmitted in one or more unique sequences and modulating the one or more unique sequences thereby creating one or more unique modulated sequences. The system and method may also include selecting for transmission one of the unique modulated sequences based on its associated PAPR, and truncating amplitudes of the selected sequence which are outside a predetermined range to thereby form a data signal that reduces power required to transmit the signal.
It is yet another object of the present disclosure to present a novel transmitter for transmitting data with multiple carriers. A transmitter may have a modulator for modulating multi-carrier symbols with the data, a processor for measuring the PAPR of the modulated data, and a logic device for comparing the PAPR with a threshold. The transmitter may also have a processor for deterministically re-sequencing the data and an amplitude filter for reducing peaks of the modulated data signal that are outside a predetermined range.
The present disclosure presents a predictive signal producing method that effectively levels transmitter output power, and results in approaching amplifier performance normally associated with constant carrier waveforms of the prior art. This solution offers >10 dB reduction in the peak-to-average power required to support the transmission of OFDM modulation techniques. This approach maximizes PAPR reduction with selective mapping and soft clipping combined. The approach also minimizes overhead, bit error rate, retransmissions, and increases latency as well as implementing processing cycles with a number of iterations. The disclosed approach improves the total system DC power efficiency and provides an optimal solution for PAPR reduction in OFDM and is uniquely different from the prior art.
These and many other objects and advantages of the present disclosure will be readily apparent to one skilled in the art to which the disclosure pertains from a perusal of the claims, the appended drawings, and the following detailed description of the preferred embodiments.